The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
An aircraft is driven by several turbojet engines each housed in a nacelle accommodating an assembly of additional actuating devices relating to its operation and ensuring various functions when the turbojet engine is in operation or stopped.
A turbojet engine nacelle generally has a substantially tubular structure which extends along a longitudinal axis from upstream to downstream along the flow direction of the air flow.
The nacelle comprises an air inlet upstream of the turbojet engine, a median section intended to surround a fan of the turbojet engine, a downstream section surrounding the combustion chamber of the turbojet engine, intended to channel the secondary air flow and optionally integrating thrust reversal means, and is generally ended by an ejection nozzle whose outlet is located downstream of the turbojet engine.
The air inlet comprises in particular an annular inlet lip forming a leading edge which is adapted to allow the optimal collection of the air necessary for supplying the fan and the inner compressors of the turbojet engine. Downstream of the lip, an acoustic downstream structure properly channels the air towards the blades of the fan.
This downstream structure of the air inlet comprises in particular an annular conduit whose structure is particularly adapted to attenuate the noise emitted by the turbojet engine also called an acoustic air inlet shroud.
For this, the shroud is mainly constituted by an acoustic sandwich structure comprising, an acoustically permeable skin also forming the inner skin of the air conduit, a sealed outer skin, and an alveolar core of predefined height linking the two skins, this core being generally of the honeycomb type, the assembly constituting an sound damper.
The shroud is assembled in its downstream portion with the fan casing along a flanging plane by means of a device including fasteners linking the flanges of the air inlet shroud and of the casing opposite to each other.
For safety reasons, in case of loss of a blade of the fan, the flanging plane must be positioned upstream of the blade of the plane of the fan.
In addition, it is desirable to attenuate the noises emitted by the engine upstream and downstream of the flanging plane and to maximize the acoustically efficient inner surface of the flow path, and in particular the acoustic surface of the air inlet shroud.
For this purpose, the document FR-A-2767560 describes and represents a device including a first L-shaped flange secured to the fan casing and a second L-shaped flange whose axial base is riveted on the outer skin of the shroud and whose radial collar is fastened to the first flange.
This type of device has in particular the disadvantage of having to link the inner skin and the outer skin of the shroud in order to associate them in one single skin with the fastening area of the second L-shaped flange.
In addition, the interlocking of an upstream portion of the casing in the shroud may be delicate to carry out and may cause a vibratory instability of the upstream portion of the casing which is overhanging. It also increases the risks of end damage of the parts during the interlocking.
Document FR-A-2869360 which proposes a second add-on L-shaped flange fastened to the outer skin of the shroud is also known. This type of device has the disadvantage of perforating the outer skin. The quality of laying the fasteners is difficult to check and the acoustic cavity between outer skin and inner skin is degraded reducing the efficiency of the area. This document also proposes an L-shaped flange directly bonded or made on the back of the outer skin of the shroud, however this device presents the issue of punching the alveolar material linking the inner skin and the outer skin during the mechanical deformations induced by the operation of the turbojet engine.
A device described and shown in the document FR-A-2959726 which includes a first flange which is secured to the casing and a second flange which is secured to the shroud is also known.
This device provides advantages by proposing an L-shaped flange linked to the shroud and which is shifted to the back of the outer skin of the shroud which avoids impacting the acoustic treatment of said shroud, but is difficult and expensive to carry out.